A toner to be used in an electrophotographic image forming apparatus usually contains inorganic or organic particles called as an external additive on the surface of the toner particles for attaining suitable image formation. The toner is designed so as to hold the properties of toner such as the charging property and fluidity by the influence of the external additive.
The compounds to be used as the external additive include a titanate compound typified by calcium titanate and strontium titanate. It has been known that the use of the titanium compound as the external additive in the toner is effective for preventing occurrence of filming on the photoreceptor surface and improving the cleaning suitability of the toner.
Moreover, it is considered that the titanate compound contributes to improving the charging property since the titanate compound has high dielectricity. Consequently, it is tried hitherto to improve the charging property of the toner. For example, a technique is known in which a negatively chargeable magnetic toner containing metal titanate particles is used to reduce the consumption of the toner by lowering the coagulation between the toner particles by the high dielectricity of the metal titanate particle; cf. Patent Publication 1. Besides, another technique is known in which stability of charging property is realized by increasing the charging property of small diameter toner particles by specifying the variation coefficient of the shape coefficient and number variation coefficient of number distribution particle size and adding the titanate compound as the external additive; Patent Publication 2.    Patent Publication 1: Tokkai Hei 8-334918    Patent Publication 2: Tokkai 2001-290302